Set the number of backfill items only when we actually begin backfill

If we get the number of items in then constructor then it is
possible that their might be more items when we actually begin
backfill. This causes a problem when the num items was 0 in the
constructor and then non-zero when we act.

Change-Id: I81b039e0a78ecfe289904bbe13e60c927eaa39cc
Reviewed-on: http://review.couchbase.org/37020
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>